A thermometer is an instrument for measuring temperature by measuring a thermodynamic property of an object, specifically by measuring variation in resistance according to thermal expansion or temperature, or by measuring thermoelectromotive force varied according to temperature, and the thermometer, in particular an industrial thermometer requires high accuracy. Industrial fields using the thermometer are a power plant, a large storage, a steel industry such as a coke plant and a plating plant, and etc, and the thermometer is used in various industrial fields.
In the case that the thermometer is used in an industrial installation as above mentioned, the thermometer is required to have accurate performance as temperature is necessary to be displayed at all times in the industrial installation and a defective product may be manufactured even by a minute error in temperature, therefore regular check for the thermometer should be carried out. However, in a conventional industrial thermometer, there is a disadvantage of great loss due to holding of process as the thermometer is detached from the industrial installation and transferred to a laboratory so as to be checked in order to calibrate the thermometer.
The calibration of the thermometer is largely divided into a method using a reference thermometer and a method using a fixed-point, and the present invention relates to the method using a fixed-point; as an example of a thermometer calibration apparatus using the fixed-point, FIG. 1 is a sectional view showing a conventional fixed-point cell and FIG. 2 is a schematic diagram showing full system of a thermometer calibration apparatus using the fixed-point cell in FIG. 1. As shown, the conventional calibration apparatus includes a fixed-point cell 10 having a thermal contact with a thermometer which is contactably inserted therein and enclosing a reference temperature material layer 20 which represents a reference temperature on the basis of a phase transition of coexisting two-phases or three-phases; a heater 30 into which the fixed-point cell 10 is inserted and heated so as to derive the phase transition; a temperature indicator 40 connected to a lead line of the thermometer for displaying a temperature measured by the thermometer with respect to the reference temperature material layer 20; a computer 50 electrically connected to the temperature indicator 40 in which reference temperatures and reference deviations with respect to each constant temperature section are stored so as to compare a difference between the reference temperature of the constant temperature section formed by the phase transition and the measured temperature and to the reference deviation, and decide whether the thermometer has detects according as whether the difference exceeds the reference deviation or not.
However, the conventional calibration apparatus using a fixed-point as above mentioned is provided with one reference temperature material layer and thus there are disadvantages that great time should be taken for measuring temperatures of wide region of many points as it takes about two days for full calibration in which a thermometer is heated by the heater of the calibration apparatus and then cooled for measuring temperature of one region, and a temperature error due to external heat is generated. Further, there exists disadvantage that in an industrial field in which the thermometer is used, manufacturing cost is increased by holding of process as it takes great time to calibrate the thermometer.